An electrical power transmission network typically comprises an electrical power generation source that is connected to an electrical power distribution network by overhead electrically conductive cables suspended between spaced-apart support towers that are installed along electric utility right-of-ways. These electrically conductive cables are susceptible to lightning strikes because of the conductive characteristic of the cabling and the height of the support towers. Cables called "groundwires" are typically suspended between the spaced-apart support towers and above the base electrical conductors of the power transmission network to protect from the high current surges presented by direct or near-by lightning strikes. These groundwires, also called shield wires or earth wires, provide a path for the high current transients generated by lightning strikes within the proximity of the groundwire to safely discharge via the groundwire, the local support towers, and the ground.
The electric utility right-of-ways for overhead electrical power transmission lines often provide an attractive path for the installation of overhead telecommunications cables. Because the communications content of light signals carried by optical fibers are not affected by the high voltage and current environment typically found within an electrical power transmission network, groundwire cables are often combined with an optical fiber or, more often, a bundle of optical fibers, to efficiently provide lightwave communications via the existing overhead transmission network. More specifically, a bundle of optical fibers are typically mounted within an electrical conductor to form a groundwire cable that is installed between spaced-apart support towers and above the electrical transmission lines. In this manner, the groundwire cable functions as both a groundwire and a telecommunications cable and thereby enables the existing electric utility right-of-way to be used for telecommunications.
A dual-purpose groundwire cable, which provides both ground fault protection and a telecommunications link, must be tolerant of the high tensional and vibrational forces presented by the overhead cable installation. More specifically, both the electrical conductor and, to a lesser extent, the bundle of optical fibers provided by the dual-purpose groundwire must be capable of withstanding the stresses presented by the overhead groundwire cable installation. Optical fibers sometimes contain defects, known as Giffith flaws, that are undetectable by the manufacturer. The flaws can lead to communications-interrupting fiber breaks upon the application of sufficient strain on the fiber.
Conventional dual-purpose groundwires provide some form of a mechanical mechanism to decouple the conductor strain from the optical fiber strain to relieve any potential optical fiber-damaging stresses during and after installation of the groundwire. In addition, the groundwire cable also must be capable of withstanding the fault currents provided by lightning strikes and the hazards provided by weather extremes, including ice, wind, and rain.
Attempts have been made in the art to protect fibers from tensile stresses by simply twisting them into helices thus increasing their lengths relative to the cable lengths. U.S. Pat. Nos. 3,955,878, 4,388,800, 4,389,088, and 4,491,386 are examples of this approach. Typically, single fibers are laid directly into channels in the cable core so that when the cables are stretched, the extra length of the fibers prevents transmission of cable elongation to the fibers. However, in all of these patents the fibers are free to move relative to the core of the cable.
Another known cable that provides both an electrically conducting ground or static wire and a fiber optic cable is described in U.S. Pat. No. 4,944,570 to Oglesby et al. This cable includes a central core that has one or more helical channels of a given twist direction or lay formed in the periphery of the core. One or more tubes containing a suitable dielectric water-blocking compound and optical fibers are positioned in the helical channels, one tube per channel. The water-blocking compound is flexible and helps to maintain the position of the fibers in the tube, but allows the fibers to move. The fibers, one or more per tube, are randomly arranged within the tubes and the tubes, the fibers and the channels are arranged in such a way that stresses from cable elongation less than a predetermined value are not transmitted to them, thus providing an elongation window. Finally, the assembly formed of the core and the tubes or tubes is wrapped with metal wires.
Other dual-purpose groundwire cables are described in U.S. Pat. Nos. Re. 32,293 and 32,374 to Dey et al. Several embodiments of the cable in these patents include at least one stranded layer of elongate elements of metal or metal alloy, at least one elongate compartment within and extending throughout the length of the stranded body and, loosely housed in the elongate compartment, at least one separate optical fiber and/or at least one optical bundle.
As in other known approaches to reducing stress on the optical fibers, the fibers in the Dey et al. patents are free to move within the compartment. A greasy material fills the interstices between the stranded layer elements to provide for relative sliding movement between stranded layers and to prevent water from entering the interior of the cable.
In the known fiber optic groundwire cables where the fibers are loosely housed and/or otherwise free to move, the strain that may be imposed upon the installed groundwire conductor is mechanically decoupled from the strain upon the supported fiber optic cable because the fiber optic cable is loosely housed within the cable core; specifically, limited relative movement between the fiber optic cable and the groundwire body can occur upon the application of tensional forces along the groundwire cable. Also, many prior art groundwire cables include a filling compound that surrounds the fiber optic cable mounted within the channel but allows the fiber optic cable to move relative to the groundwire electrical conductor.
Despite the mechanisms provided in the prior art for decoupling the electrical conductor strain from the optical fiber strain, the tensional and vibrational forces upon the typical cable limits the estimated expected lifetime of the optical fibers that form the fiber optic cable within the electrical conductor. Also, the dual-purpose groundwire cables provided by the prior art are also limited by the maximum number of optical fibers carried by the cable because of the loose positioning of the fibers within the electrical conductor--the provision of space for movement limits the number of optical fibers that can be present.
Many optical fibers used for telecommunications today are manufactured with a coating of acrylate to microcracks in the glass fibers formed during extrusion and prevent the incursion of moisture. The acrylate coating on most fibers is extremely thin and provides no cushioning from chafing or from compressive forces generated by contact with adjacent fibers in a bundle or with the surfaces of a channel. Moreover, at temperatures of greater than about 180.degree. C., acrylate melts and the fiber's moisture protection provided by the coating is degraded. Groundwires made with acrylate coated fibers therefore suffer from a relatively low rating of fault current carrying capability, since current above the rating will elevate the temperature and degrade the optical fibers, ultimate resulting in shorter fiber life.
Therefore, there is a need for a dual-purpose fiber optic groundwire cable that demonstrates an improvement in the estimated expected lifetime of the optical fibers located within the electrical conductor. A need further exists for a more efficient dual-purpose fiber optic groundwire cable that carries more optical fibers than can be carried by prior art approaches. In addition, there is a need for a fiber optic groundwire cable that provides an improved maximum fault current capacity. The groundwire cable must be able to withstand the tensional and vibrational forces provided by the typical overhead cable installation, and must be tolerant of extreme temperature ranges and the hazards provided by all kinds of weather, including ice, wind, and rain.